In Episode 28: "Teddy and the Mudblups",
Teddy,Grubby
and Gimmick find themselves at the
all-too-familliar Mudblup caves.
They are re-captured and Teddy comes up with an idea to get them
out. They cover themselves in mud to look like Mudblups and find
their way to the entrance. When they come out in the open, it
begins to rain. The mud washes off of them and the Mudblups realize
they've been tricked. Teddy and Gimmick make it to the Airship
safely, but Grubby is slow and doesn't realize the Mudblups are
hot on his tail.

Gimmick pulls out a crystal
and aims it at Grubby. Placing it on a tray and refracting the
light off of the crstal and onto Grubby, the effects of the crystal
are not what he planned as Grubby begins to slow down. Teddy
suggests turning the crystal around and trying again. When Gimmick
does this, the cyrstal speeds him up and he just escapes the
clutches of the Mudblups as the sun peeks over the horizon.

This solves the mystery of the fifth crystal.
It allows one to go slow or fast, depending upon which way it
is facing when the light is refracted from it. This comes in
handy many times throughout the series when the trio find themselves
in situations such as these.